One of the stars that defines the summer season crowns the sky early this evening. Vega stands directly overhead for those at the middle latitudes of the United States, and close to overhead for those who are farther north or south.

Vega’s the second-brightest star in summer evening skies as seen from the U.S. Only Arcturus, which is well up in the west at nightfall, outshines it. Vega’s also the leading light of one of the signature star patterns of summer evenings, the Summer Triangle.

Vega shines so brightly for a couple of reasons. First, it’s quite close — just 25 light-years away. Only a couple of hundred known stars are closer. And second, Vega really is a bright star — it’s in the top few percent of all the stars in the galaxy.

Vega’s true brightness is a function of its size and temperature. It’s more than twice the diameter of the Sun, so there’s a lot of surface area to radiate light into space. And Vega’s surface is several thousand degrees hotter than the surface of the Sun. A hotter surface emits more light than a cooler surface of the same area.

The size and temperature are in turn driven by the star’s mass and its phase of life. Vega’s about twice as heavy as the Sun, so the nuclear reactions in its core take place at a much faster rate. And Vega’s in the “main sequence” phase of its life, steadily converting its original hydrogen fuel to helium — a process that keeps the star shining brightly.